Apparatus for cleansing flue gases

ABSTRACT

The invention relates to an apparatus for purifying the gases to remove sulphur dioxide. The apparatus comprises a container for a body of treatment liquid and is provided with a gas inlet so arranged that the gas is caused to flow substantially at right angles to the surface of the liquid therein. A treatment passage extends substantially vertically upwards from the liquid surface in the container, so that the gas after contact with the liquid flows substantially linearly therethrough entraining droplets of liquid. The container communicates below the liquid level with an equalizing tank having means for introducing alkali thereinto to maintain the treatment liquid at a suitable alkalinity and with means for introducing fresh water. The equalizing tank also has means to maintain the liquid level therein constant whereby the liquid level in the treatment container adjusts automatically to variations in the rate of gas supplied thereto. The invention also includes means in the equalizing tank for accelerating the admixture of powdered alkali in the liquid.

United States Patent [15] 3,640,053 Gustavsson Feb. 8, 1972 [54]APPARATUS FOR CLEANSING FLUE 2,792,905 5/ 1957 Forrest ..55/227 GASES3,315,443 4/1967 Marino ....55/228 3,527,026 9/1970 Miura ..55/239 [72]Inventor: Karl-Axel Goran Gustavsson, Enkoping,

Sweden FOREIGN PATENTS OR APPLICATIONS [73] Assignee: AktienbolagetBahco Ventilation, Enkop- 1,191,954 5/1970 Great Britain ..55/84 ing,Sweden Primary Examiner-Remard Nozick [22] Filed. June 1, 1970 AtmmeyHarry Cohen [21] Appl. No.: 42,231

[57] ABSTRACT R l ted U.S. Ii ti D ta App ca on a The invention relatesto an apparatus for purifying the gases to contmuatwn-ln-part of 3611 Premove sulphur dioxide. The apparatus comprises a container 1968,abandoned. for a body of treatment liquid and is provided with a gasinlet so arranged that the gas is caused to flow substantially at right[30] Foreign Application Priority Data angles to the surface of theliquid therein. A treatment passage extends substantially verticallyupwards from the liquid sur- June 2, 1969 Sweden ..7759/69 face in thecontainer, so that the g after Contact with the liquid flowssubstantially linearly therethrough entraining [52] us 2 droplets ofliquid. The container communicates below the [51] Int. Cl Bold 47/02liquid level with an equalizing tank having means for introduc- [58]Field 95 225 ing alkali thereinto to maintain the treatment liquid at asuita- 55/226 E 4 4 ble alkalinity and with means for introducing freshwater. The 339 261/79 119 23/225 1 equalizing tank also has means tomaintain the liquid level therein constant whereby the liquid level inthe treatment container adjusts automatically to variations in the rateof gas supplied thereto. The invention also includes means in the [56]References cued equalizing tank for accelerating the admixture ofpowdered al- UNITED STATES PATENTS kali in the q 2,l42 ,406 l/ 1939 Nonhebel et a1 ..55/228 2 Claims, 2 Drawing Figures PATENTEIJ FEB 8:972

sum 1 or 2 Fig. 7

PATENTED FEB 8872 SHEET 2 BF 2 APPARATUS FOR CLEANSING FLUE GASES Thisapplication is a continuation-in-part of our copending application Ser.No. 763,424 filed on Sept. 27, 1968 now abandoned.

The present invention is concerned with an apparatus for removingimpurities, primarily S and secondarily solid products of combustionsuch as soot, coke dust and ash, from flue gases.

The discharge of flue gases from different firing plants, e.g., oilfiring plants, and waste gases from industrial processes represents aproblem of increasing magnitude, mainly because of the quantities of S0and soot which impurify the air as a result thereof. Consequently, it isbecoming more and more urgent to find ways and means for the removal ofsuch impurities from the flue gases in an effective and economic manner,in order to prevent the damage which such impurities cause, such ascorrosion, poisoning of vegetation, smog formation etc. From the pointof view of economy the discharge of SO would seem to be the most harmfulof these impurities, as a result of the damage it causes due tocorrosion.

--The sulphur dioxide can be absorbed on solid substances having a widespecific surface, e.g., activated carbon or some other strongly expandedmaterial, but the decontamination of these materials with soot andabsorption of steam from the gases of combustion greatly impairs theirutility in practice, since they would require an extensive exchangeservice and servicing program, resulting in too high costs.

It is also possible to cleanse the oil from sulphur in connection withthe production process, but this would cause the price of oil to rise tosuch an extent that this possibility cannot be accepted.

A third method of removing sulphur impurities from flue gases is basedon the well-known fact that the S0 is readily absorbed in alkalineaqueous solutions. Two problems associated herewith, however, are firstthat a sufficient contact surface between the alkaline treatment liquidand the S0 bearing gas must be obtained, and secondly the length of timeduring which the gas and liquid are in contact must be sufficient toallow for practically 100 percent absorption.

It is possible to obtain a wide contact surface by atomizing thealkaline treatment liquid. An obvious expedient for this purpose is theuse of nozzles working under pressure. However, nozzles are prone tobecome blocked, worn and corroded, particularly when working withsludge-bearing liquids. A more advantageous effect is obtained with anapparatus in which the liquid is atomized or broken down by causing thegas to attack the surface of the treatment liquid at high speed. The gasthen carries the drops of water up through a vertical treatment passage,the lower end of which terminates immediately above the surface of theliquid, and in which the absorption process is effected.

The other requirement to be fulfilled is one of sufficient contact time.This can be provided for by successively reducing the velocity of thegas during the flow upwards through the vertical treatment passage,which is designed as a diffusor for this purpose. in this way anappreciable portion of the kinetic energy of the gas is also recovered,and can be applied to break down the liquid.

The treatment passage or column is kept filled during the treatmentprocess, with drops of the treatment liquid or absorption liquid itself.Thus, this column differs to advantage from such columns which in orderto provide for the requisite contact surface are filled with packingbodies, which are overrun with liquid passed by a gas flowing slowlythrough the column, whereby very heavy structures result and the bed ofpacking bodies tends to become blocked with solid impurities in the gasor with the absorption agent.

By maintaining a suitable liquid level, gas velocity upon liquidentrainment and the angle at which the passage widens out it is possibleto obtain very good filling of the passage with suspended droplets whichabsorb S0 and which by depositing on the walls and being recirculated tothe liquid bath can also lead away the absorbed S0 One problemassociated with an apparatus of the aforementioned type, however, isthat the treatment process is sensitive to variations in the distancebetween the level of the liquid in the container and the lower end ofthe passage, which is a decisive factor for the size of the dropletsentrained by the gas, and hence for a given space velocity of the gas atruly effective effect is only obtained within a very narrow range forsaid distance. The process is thus difficult to control in the mannerdesired. Furthermore, it has been found than an apparatus of givendimensions functions best within a rather small range for the gassupply.

To enable the absorption arrangement to be controlled more easily and,at the same time, better suited for use with widely varying gas supplyit is possible, according to the previous suggestion, to arrange anannular slot at the lower end of the treatment column, at such a heightabove the normal level of the liquid that an essentially liquid-freepart flow of gas passes through the slot. The effect of this annularflow of gas on the liquid droplets is to break down the droplets in thetreatment column into considerably smaller droplets, whereby the amountof liquid which exists in droplet form in the treatment column increasesconsiderably. The apparatus is thus less sensitive to variations in theliquid level and, at the same time, also functions well with a morevarying gas supply. The portion of the outlet passage below the slot canbe produced in the form of a vertically movable connection pipe whoseposition vertically can be adjusted. The connection pipe can be arrangedfreely movable so that it automatically adjusts the width of the slotunder the influence of the gas flow. However, it has been found thatalthough this arrangement functions satisfactorily for a relatively longperiod of time it does in the course of time become less effective inapparatus for separating sulphur dioxide with the aid of alkali,particularly when the alkali is lime since sludge deposits form on thestationary sur- 7 face of the apparatus of the slot, and thereby impairsits function.

The object of the present invention is to provide for automaticadjustment of the said liquid level in relation to the lower end of thetreatment column under the influence of variations in the gas flow, sothat the apparatus automatically adjusts itself to the prevailing gasflow, without the use of movable parts whose function might be impairedby blockages. This object is achieved by means of the present inventionin that the container which houses the body of liquid and from which thetreatment column extends communicates below the level of the liquid withan equalizing container or surge tank, the liquid level of which is heldconstant. Upon variations in the gas flow, and the subsequent changes inpressure above the body of liquid as a result thereof, the level of theliquid below the lower end of the treatment column will rise or fall, inaccordance with the known principle for communicating vessels, so thatthe level of the liquid adjusts itself in response to the gas flow.

The invention is thus concerned with an apparatus for cleansing flue gasfrom primarily S0 and includes a container adapted to accommodate aquantity of treatment liquid and provided with a gas inlet so arrangedthat the gas is caused to flow substantially at right angles downtowards the surface of the liquid in the container; and a treatmentcolumn directed essentially perpendicularly to the surface of the liquidand through which the gas flows essentially linearly away from thesurface of the liquid while entraining droplets of liquid therefrom, andwherein the said container below the liquid level communicates with anequalizing tank provided with means for maintaining the level of theliquid therein constant.

Among the alkalis which can be used as treatment or absorption agentsare primarily sodium hydroxide (caustic soda) and calcium hydroxide(hydrated lime). The caustic soda is obviously a more superiorabsorption agent, but since it is much too expensive aqueous calciumhydroxide is generally used.

The hydrated lime may conveniently be metered direct in powder form tothe equalizing tank, which may thus simultaneously serve as a dissolver,by means of a screw conveyor or other mechanical devices. The strongcirculation of water effects the mixing and dissolving of thesubstances. Metering of the lime can also be controlled by instrumentsfor controlling the pH value of the return liquid from the top of thetreatment column, so that an alkali of pH 7-1 1 is maintained there. Thelime solution thus becomes supersaturated in remaining parts of thesystem, and conventional iron plate can be used as construction materialwithout too much risk for corrosion.

The only waste product is the sludge formed by the consumed hydratedlime and solid constituents and sulphur compounds washed from the fluegas. The sludge can be tapped off intermittently, through a pocket underthe liquid bath. However, it is desirable to obtain a higherconcentration than that obtained by self-sedimentation, and accordinglya fully automatic operating sludge thickener can be connected in thereturn circuit for the return liquid so that the sludge is discharged inthe form of a concentrated paste while the liquid freed from sludge ispassed back to the liquid bath. This liquid may suitably be returned sothat it flushes the walls of the container free from any sludge whichmight have settled thereon.

The apparatus may include more than one treatment stage, wherewithsubsequent to its exit from the treatment column and the dropletseparating process the gas treated in the manner described above iscaused to attack the surface of another liquid in a second container,and is passed through a further treatment column. In this instance alevel regulator of the above-described type is also incorporated in thesecond stage. The two equalizing tanks may also be connected together ina suitable manner.

The equalizing tank also serves as a dissolver for the powdered solidalkali and the present invention also provides means for facilitatingthe introduction of powdered alkali into the liquid in' the equalizingtank.

For the aforementioned purpose, there has hitherto been employed a tankinto the upper portion of which lime is fed by means of a screw conveyorand caused to fall down directly onto the water contained in the lowerportion of the container. It has been found with such an apparatus thatthe function of the screw conveyor is seriously impaired by the factthat water vapor from the lower portion of the container enters theconveyor and causes the powdered lime therein to agglomerate.

The present invention intends to avoid this disadvantage and toconstruct the apparatus in a manner which hastens the mixing of thepowder in the liquid. Accordingly, the apparatus is mainly characterizedin that mounted in the upper portion of the apparatus is an inner liquidcontainer which is con nected to a conduit for supplying liquid thereinand communicating with the outer container through a vertical tube, theupper end of which is placed over the bottom of the inner container andso that the outlet of themeans feeding the powdered material is locatedimmediately thereabove.

With this apparatus a downwardly passing flow of liquid is produced,which tends to drive water vapor arriving from the lower portion of theapparatus away from the area at which the powdered material is chargedto the apparatus, so that no steam can reach the charging means. Whenthe powdered material is fed directly into the liquid flow an effectivemixing of the powder with the liquid is obtained, which contributestowards rapid dissolving of the powder.

The invention will now be described in more detail with reference to anembodiment thereof illustrated in the accompanying drawing, in whichFIG. 1 is a flue gas cleansing apparatus constructed in accordance withthe invention, and

P10. 2 is a cross-sectional view of the upper portion of the limedissolving equalizing tank shown in F 1G. 1.

The flue gas cleansing means shown in FIG. 1 functions in accordancewith the principle whereby a flow of gas is directed at right anglesdown towards a body of liquid and then made to flow substantiallyvertically upwards from the body of liquid and then made to flowsubstantially vertically upwards from the body of liquid, whileentraining droplets of liquid, and through a treatment chamber, in whichthe gas and liquid are held in contact with each other for a long periodof time. The liquid droplets are separated from the gas at the upper endof the treatment chamber and returned to the body of liquid while thepurified gas leaves the chamber. A lime solution is used in theapparatus as an absorption liquid for taking up sulphur dioxide. Thelime solution is prepared by introducing to a lime dissolving means,which communicates with the container holding the body of liquid, aquantity of powdered lime corresponding to the quantity consumed (byforming a calcium sulfite 1n the shown embodiment the gas is treated intwo stages and the treatment apparatus (the scrubber) is thereforeprovided with a lower liquid container 1,, a lower treatment chamber 2,a lower droplet separator 3, an upper liquid container 4, an uppertreatment chamber 5 and an upper droplet separator 6.

The lower liquid container 1 contains absorption liquid up to a level 7,which while the apparatus is in operation is maintained at a smalldistance below the lower end of the treatment chamber 2, which comprisesan upwardly slightly conical, expanded tube whose lower part projectsdown into the container 1.

The gas to be cleansed is fed by means of a fan 8 through a line 9 intothe upper portion of the container 1 in a manner whereby it flowssubstantially at right angles down against the surface 7, whereafter itcontinues up through the chamber 2. The gas thus breaks the liquid atthe surface into fine droplets, which are entrained with the gas throughthe treatment chamber, which is therefore constantly filled with amixture of gas and liquid droplets. The liquid absorbs in the treatmentchamber any solid impurities contained by the gas land sulphur dioxidetherefrom.

The mixture of gas and liquid droplets is suitably caused to rotate bymeans of blades 10 before entering the drop separator 3, where theliquid is separated and returned to the liquid container 1, the liquidpassing through the chamber 2, a conduit 11 and a settling tank 12connected to the conduit 11 by means of a branch conduit 13.

The described treatment sequence is repeated in the body of liquid inthe container 4 arranged at the upper portion of the drop separator 3,the treatment chamber 5 and the droplet separator 6, wherefrom thecleansed gas departs through the outlet 14 and the liquid returnsthrough a conduit 15 to the container 1.

The container 1 communicates via a conduit 16 with an equalizing andlime dissolving tank 17 which serves at the same time as a levelregulating vessel, the liquid level 18 in the vessel 17 being maintainedconstant for the purpose of controlling the level of the liquid in thecontainer 1. The tank is also provided with a liquid supply conduithaving a valve 24. The container 17 may, for instance, be provided witha float which opens the valve 24 when the liquid level 18 tends to fall,and also with a spillway (not shown) by way of which the liquid departswhen the level rises. The lower portion of the tank 17 communicatesthrough a conduit 19 having a pump 20 with an upper equalizing tank 21,which communicates with the upper container 4 through a conduit 11 andwhich has a vent opening to the atmosphere. The liquid in the vessel 21is maintained at a constant level by means of a spillway 23, whichpasses the liquid back to the lime dissolving tank 17.

Liquid is passed to the lime dissolving tank from the settling tank 12through the conduit 25.

The sludge formed when the apparatus is in operation (the sludgecomprising calcium sulflte, calcium sulfate, calcium carbonate etc.)settles partly in the container 1 and partly in the settling tank 12,the major part of the sludge settling here, and can be tapped off to asludge tank 28 through bottom outlets 26, 27 provided with valves.

In order to replace the lime consumed during the process, powdered limeis fed by means of a screw 30 from a bin 29 to the lime dissolving means17. The feed can be regulated, for

example, by determining the pH of the liquid in the conduit 13 or insome other manner.

The temperature of the liquid, when the apparatus is in operation isrelatively high. Heat is also developed when the lime is dissolved inthe liquid. As a result steam is formed in the lime dissolving tank 17.lt has been discovered that, when the apparatus is in operation, thesteam is liable to cause disturbances in the work of the screw 30 sincethe powdered lime is baked into agglomerates therein. To avoid this, thelime dissolving tank has been given the construction illustrated in FIG.2.

Arranged in the upper portion of the lime dissolving tank is a liquidcontainer 31 which is connected to the conduit 23 extending from thevessel 21, whereby return liquid from the vessel flows into thecontainer 31. The container 31 is open at the top thereof, at 32, andthe outlet 33 of the screw is placed in this opening. The container 31also has an opening 34 arranged in the bottom thereof and positionedimmediately beneath the outlet 33. Extending through the opening 34 is atube 35, the lower end of which is situated slightly above the liquidlevel 18 in the lime dissolving means 17. The upper end 36 of the tubeis funnel shaped and a number of recesses 37 can suitably be arranged inthe upper end of the funnel.

When the apparatus is in operation, liquid flows from the conduit 23 tothe container 31 and over the upper edge of the funnel 36 down in thetube 35 to the main part of the lime dissolving means. This flow ofliquid creates a downward flow of air through the conduit 35 whichdrives the steam away, causing the steam to pass to atmosphere throughthe outlet 38. Consequently no steam will reach the screw 30. Thepowdered lime is fed down into and received by the liquid flowingthrough the tube, with which it is rapidly mixed.

The outlet 37 serves to concentrate the flow of liquid to determinedpoints around the periphery of the tube, thereby further improving theeffect.

I claim:

1. An apparatus for cleansing flue gases, primarily 80,, which includesa container adapted to accommodate a quantity of alkaline treatmentliquid, gas inlet means communicating with the container and arranged sothat the gas is caused to flow substantially at right angles downtowards the surface of the liquid in the container, a treatment passageconnected to the container and directed essentially perpendicular to thesurface of the liquid and through which the gas flows essentiallylinearly away from said surface of the liquid while entraining dropletsof liquid therefrom, a droplet separator connected to the outlet of thetreatment passage, an equalizing and alkali dissolving tankcommunicating with said container below the liquid level therein, asettling tank means for removing solid material from the treatmentliquid, conduit means providing communication between said dropletseparator and said settling tank means for passing treatment liquid tosaid settling tank means, conduit means providing communication betweensaid settling tank means and said container for returning liquid fromsaid settling tank means to said container, means for maintaining aconstant liquid level within said equalizing and alkali dissolving tank,said means including a liquid inlet and a liquid outlet, and solidmaterial fccding means in communication with said equalizing and alkalidissolving tank for supplying alkali in solid form to the liquid in thelatter to maintain the alkalinity of said treatment liquid.

2. An apparatus as claimed in claim 1, including, in the upper portionof said equalizing and alkali dissolving tank, an inner liquidcontainer, which communicates with the outer tank through a verticaltube, the upper end of which is located above the bottom of said innercontainer and vertically below the outlet of said solid material feedingmeans.

2. An apparatus as claimed in claim 1, including, in the upper portion of said equalizing and alkali dissolving tank, an inner liquid container, which communicates with the outer tank through a vertical tube, the upper end of which is located above the bottom of said inner container and vertically below the outlet of said solid material feeding means. 